The Mana Bar is the blue bar at the bottom in DOTA 2

Mana is the resource a hero uses to cast most of his abilities. The mana pool is represented with a blue bar below the green health bar and 2 numbers ( # / # ) in the middle of it. The first one is your actual quantity of mana and the second refers to the total quantity you can store. Mana can be increased by certain items/abilities/attributes. Mana Regen is the amount of Mana gained every second.